The CDMA technique is used in particular in so-called “third generation” systems, and in particular in the universal mobile telecommunication system (UMTS).
In general, in such systems, one of the objects is to increase performance, i.e., specifically, to increase capacity and/or to improve quality of service.
A commonly used technique is the technique known as Closed loop power control.
The purpose of closed loop power control is to maintain some parameter representative of the quality of transmission over a link as close as possible to a target value for each of the links between a base station and a mobile station (where said parameter can be the signal to interference ratio (SIR)). For example, in the downlink direction (i.e. from base stations to mobile stations), a mobile station transmits power control commands to a base station for the purpose of reducing the difference between the SIR as estimated by the mobile station and the target SIR value. For better effectiveness in said internal power control loop, such power control commands are transmitted quite often; by way of example, in a system such as the UMTS in which transmitted information is structured as frames, frames themselves being built up from time slots, a power control command is transmitted in each time slot. The target value can itself be determined by another control loop for maintaining a constant quality of service, said second loop generally being referred to as an “outer” loop, as compared with the preceding loop which is also referred to as an “inner” loop.
Another technique in widespread use in CDMA systems is the technique known as macro-diversity transmission or else as “soft” handover, in which a mobile station is connected simultaneously to a plurality of base stations. By using suitable techniques for processing and combining the various signals received from the various base stations by the mobile station (in particular by means of a “rake” type receiver), this makes it possible to improve performance on reception, and also to minimize the risk of a call being lost during transfers between cells, unlike the “hard” handover technique in which a mobile station is connected at any one instant to a single base station only.
In general, when using macro-diversity transmission, the inner power control loop in the downlink direction is such that the mobile station transmits the same power control command at any one instant to each of the base stations with which it is connected.
When using marco-diversity transmission, it is also known to adjust the transmission power levels of the various base stations to which a mobile station is connected relative to one another so as to optimize the performance of the system, and in particular so as to optimize its capacity. Thus, in general, a radio network controller (RNC) in the UMTS system which serves to control base stations determines the power to be used for transmission at an instant tn+1 for each of the base stations on the basis of the transmission powers at an instant tn as reported by each of said base stations. Advantageously, the same transmission power is determined for the various base stations.
The transmission power value(s) as determined in this way (also referred to as reference transmission power(s)) are then signalled by the radio network controller to the various base stations which respond by adjusting their transmission powers on said reference transmission powers.
Furthermore, document WO 99/31819 proposes that the radio network controller informs each base station with a synchronization instant (which is the same for all base stations) simultaneously with the reference transmission power, so as to adjust transmission powers simultaneously amongst the various base stations.
An initial reference transmission power value can be determined on each occasion that one or more base stations are added or removed from the set of base stations with which a mobile station is connected, as the mobile station moves (which set is also referred to as the “active” set). Updating said reference transmission power value can then be necessary in the event of drift from the initial scheme, in particular in the event of power control command transmission errors in the inner power control loop, causing a given power control command to be transmitted no longer to all of the base stations in an active set, in which case the various transmission powers are no longer properly adjusted relative to one another.
Document WO 99/31819 also proposes informing each base station of a synchronization instant on each occasion that the reference transmission power value is updated, simultaneously with the updated reference transmission power value.
That solution suffers in particular from the drawback of significantly increasing the amount of signalling interchanged at the interface between the base stations and the radio network controller (which interface is also known as the “Iub” in UMTS), and thus to cease making effective use of the transmission resources available at said interface.